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Michel Viret

Publications and source records attributed to Michel Viret.

At least 19 recordsLinked to original sources
arXiv2026

Direct spatiotemporal imaging of carriers reveals a long-lived bulk photovoltaic mechanism

The bulk photovoltaic effect (BPVE), a manifestation of broken centrosymmetry, has attracted interest as a probe of the symmetry and quantum geometry of materials, and for use in photovoltaic and optoelectronic devices. However, so far the effect has not been captured directly in space and time. Here, we use contactless pump-probe microscopy to visualize the spatiotemporal evolution of photoexcited carriers in single-crystal, mono-domain $BiFeO_{3}$, a prototypical ferroelectric material. We observe asymmetric carrier transport along the polar axis, which confirms the intrinsic bulk- and symmetry-driven nature of the BPVE. Remarkably, this asymmetric transport persists for several nanoseconds after photoexcitation, which cannot be explained by conventional short-lived shift or phonon ballistic current BPVE mechanisms. Our Monte Carlo simulations show that asymmetric momentum scattering by defects, such as oxygen vacancies, leads to long-lived asymmetric carrier transport, as observed experimentally. Beyond fundamental insights, this paves the way towards controlling symmetry- and defect-driven photoresponses.

cond-mat.mtrl-sci↗
arXiv2024

Terahertz antiferromagnetic dynamics induced by ultrafast spin currents

Insulating antiferromagnets are anticipated as the main protagonists of ultrafast spintronics, with their intrinsic terahertz dynamics and their abililty to transport spin information over long distances. However, direct transfer of spin angular momentum to an antiferromagnetic insulator at picosecond time scales remains to be demonstrated. Here, studying the ultrafast behaviour of ferromagnetic metal/antiferromagnetic insulator bilayers, we evidence the generation of coherent excitations in the antiferromagnet combined with a modulation of the demagnetization behavior of the ferromagnet. This confirms that magnetic information can indeed be propagated into antiferromagnetic spin waves at picosecond timescales, thereby opening an avenue towards ultrafast manipulation of magnetic information.

cond-mat.mes-hall↗
arXiv2023

Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range

The ultrafast and element-specific response of magnetic systems containing ferromagnetic 3d transition metals and 4d/5d heavy metals is of interest both from a fundamental as well as an applied research perspective. However, to date no consensus about the main microscopic processes describing the interplay between intrinsic 3d and induced 4d/5d magnetic moments upon femtosecond laser excitation exist. In this work, we study the ultrafast response of CoFeB/Pt bilayers by probing element-specific, core-to-valence band transitions in the extreme ultraviolet spectral range using high harmonic radiation. We show that the combination of magnetic scattering simulations and analysis of the energy- and time-dependent magnetic asymmetries allows to accurately disentangle the element-specific response in spite of overlapping Co and Fe M$_{2,3}$ as well as Pt O$_{2,3}$ and N$_7$ resonances. We find a considerably smaller demagnetization time constant as well as much larger demagnetization amplitudes of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our results are in agreement with enhanced spin-flip probabilities due to the high spin-orbit coupling localized at the heavy metal Pt, as well as with the recently formulated hypothesis that a laser generated, incoherent magnon population within the ferromagnetic film leads to an overproportional reduction of the induced magnetic moment of Pt.

cond-mat.mtrl-sci↗
arXiv2022

Imaging topological defects in a non-collinear antiferromagnet

We report on the formation of topological defects emerging from the cycloidal antiferromagnetic order at the surface of bulk BiFeO$_3$ crystals. Combining reciprocal and real-space magnetic imaging techniques, we first observe, in a single ferroelectric domain, the coexistence of antiferromagnetic domains in which the antiferromagnetic cycloid propagates along different wavevectors. We then show that the direction of these wavevectors is not strictly locked to the preferred crystallographic axes as continuous rotations bridge different wavevectors. At the junctions between the magnetic domains, we observe topological line defects identical to those found in a broad variety of lamellar physical systems with rotational symmetries. Our work establishes the presence of these magnetic objects at room temperature in the multiferroic antiferromagnet BiFeO$_3$, offering new possibilities for their use in spintronics.

cond-mat.mes-hall↗
arXiv2021

Spin charge conversion in Rashba split ferromagnetic interfaces

We show here theoretically and experimentally that a Rashba-split electron state inside a ferromagnet can efficiently convert a dynamical spin accumulation into an electrical voltage. The effect is understood to stem from the Rashba splitting but with a symmetry linked to the magnetization direction. It is experimentally measured by spin pumping in a CoFeB/MgO structure where it is found to be as efficient as the inverse spin Hall effect at play when Pt replaces MgO, with the extra advantage of not affecting the damping in the ferromagnet.

cond-mat.mes-hall↗
arXiv2021

Ultrafast time-evolution of chiral Néel magnetic domain walls probed by circular dichroism in x-ray resonant magnetic scattering

Non-collinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fueled by possible fine engineering of several magnetic interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This allows the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions. We report here on the ultrafast behavior of chiral DWs after optical pumping in perpendicularly magnetized asymmetric multilayers, probed using time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe a picosecond transient reduction of the CD-XRMS, which is attributed to the spin current-induced coherent and incoherent torques within the continuously dependent spin texture of the DWs. We argue that a specific demagnetization of the inner structure of the DW induces a flow of hot spins from the interior of the neighboring magnetic domains. We identify this time-varying change of the DW textures shortly after the laser pulse as a distortion of the homochiral N'eel shape toward a transient mixed Bloch-Néel-Bloch textures along a direction transverse to the DW. Our study highlights how time-resolved CD-XRMS can be a unique tool for studying the time evolution in other systems showing a non-collinear electric/magnetic ordering such as skyrmion lattices, conical/helical phases, as well as the recently observed antiskyrmion lattices, in metallic or insulating materials.

cond-mat.mtrl-sci↗
arXiv2020

Spin Insulatronics

Spin insulatronics covers efforts to generate, detect, control, and utilize high-fidelity pure spin currents and excitations inside magnetic insulators. Ultimately, the new findings may open doors for pure spin-based information and communication technologies. The aim is to replace moving charges with dynamical entities that utilize low-dissipation coherent and incoherent spin excitations in antiferromagnetic and ferromagnetic insulators. The ambition is that the new pure spin-based system will suffer reduced energy losses and operate at high frequencies. In magnetic insulators, there are no mobile charge carriers that can dissipate energy. Integration with conventional electronics is possible via interface exchange interactions and spin-orbit couplings. In this way, the free electrons in the metals couple to the localized spins in the magnetic insulators. In turn, these links facilitate spin-transfer torques and spin-orbit torques across metal-insulator interfaces and the associated phenomena of spin-pumping and charge-pumping. The interface couplings also connect the electron motion inside the metals with the spin fluctuations inside the magnetic insulators. These features imply that the system can enable unprecedented control of correlations resulting from the electron-magnon interactions. We review recent developments to realize electric and thermal generation, manipulation, detection, and control of pure spin information in insulators.

cond-mat.mes-hall↗
arXiv2020

Ultrafast antiferromagnetic switching in NiO induced by spin transfer torques

NiO is a prototypical antiferromagnet with a characteristic resonance frequency in the THz range. From atomistic spin dynamics simulations that take into account the crystallographic structure of NiO, and in particular a magnetic anisotropy respecting its symmetry, we describe antiferromagnetic switching at THz frequency by a spin transfer torque mechanism. Sub-picosecond S-state switching between the six allowed stable spin directions is found for reasonably achievable spin currents, like those generated by laser induced ultrafast demagnetization. A simple procedure for picosecond writing of a six-state memory is described, thus opening the possibility to speed up current logic of electronic devices by several orders of magnitude.

cond-mat.mtrl-sci↗
arXiv2018

On the magnetoelastic and magnetoelectric couplings across the antiferromagnetic transition in multiferroic BiFeO3

Clear anomalies in the lattice thermal expansion (deviation from linear variation) and elastic properties (softening of the sound velocity) at the antiferromagnetic-to-paramagnetic transition are observed in the prototypical multiferroic BiFeO3 using a combination of picosecond acoustic pump-probe and high-temperature X-ray diffraction experiments. Similar anomalies are also evidenced using first-principles calculations supporting our experimental findings. Those calculations in addition to a simple Landau-like model we also developed allow to understand the elastic softening and lattice change at T_N as a result of magnetostriction combined with electrostrictive and magnetoelectric couplings which renormalize the elastic constants of the high-temperature reference phase when the critical T_N temperature is reached.

cond-mat.mtrl-sci↗
arXiv2017

Electrical properties of single crystal Yttrium Iron Garnet ultra-thin films at high temperatures

We report a study on the electrical properties of 19 nm thick Yttrium Iron Garnet (YIG) films grown by liquid phase epitaxy. The electrical conductivity and Hall coefficient are measured in the high temperature range [300,400]~K using a Van der Pauw four-point probe technique. We find that the electrical resistivity decreases exponentially with increasing temperature following an activated behavior corresponding to a band-gap of $E_g\approx 2$ eV, indicating that epitaxial YIG ultra-thin films behave as large gap semiconductor, and not as electrical insulator. The resistivity drops to about $5\times 10^3$~$Ω\cdot \text{cm}$ at $T=400$ K. We also infer the Hall mobility, which is found to be positive ($p$-type) at 5 cm$^2$/(V$\cdot$sec) and about independent of temperature. We discuss the consequence for non-local transport experiments performed on YIG at room temperature. These electrical properties are responsible for an offset voltage (independent of the in-plane field direction) whose amplitude, odd in current, grows exponentially with current due to Joule heating. These electrical properties also induce a sensitivity to the perpendicular component of the magnetic field through the Hall effect. In our lateral device, a thermoelectric offset voltage is produced by a temperature gradient along the wire direction proportional to the perpendicular component of the magnetic field (Righi-Leduc effects).

cond-mat.mtrl-sci↗
arXiv2017

Spin conductance of YIG thin films driven from thermal to subthermal magnons regime by large spin-orbit torque

We report a study on spin conductance in ultra-thin films of Yttrium Iron Garnet (YIG), where spin transport is provided by propagating spin waves, that are generated and detected by direct and inverse spin Hall effects in two Pt wires deposited on top. While at low current the spin conductance is dominated by transport of thermal magnons, at high current, the spin conductance is dominated by low-damping non-equilibrium magnons thermalized near the spectral bottom by magnon-magnon interaction, with consequent a sensitivity to the applied magnetic field and a longer decay length. This picture is supported by microfocus Brillouin Light Scattering spectroscopy.

cond-mat.mtrl-sci↗
arXiv2015

Bismuth-based perovskites as multiferroics

This review devoted to multiferroic properties of Bismuth-based perovskites falls into two parts. The first part focuses on BiFeO3 and summarizes the recent progress made in the studies of its pressure-temperature phase diagram and magnetoelectric coupling phenomena. The second part discusses in a more general way the issue of polar - and multiferroic - phases in BiBO3 perovskites and the competition between ferroelectricity and other structural instabilities, from an inventory of recently synthetized compounds.

cond-mat.mtrl-sci↗
arXiv2014

Prediction of novel interface-driven spintronic effects

The recently-proposed coupling between the angular momentum density and magnetic moment [A. Raeliarijaona et al, Phys. Rev. Lett. 110, 137205 (2013)] is shown here to result in the prediction of (i) novel spin currents generated by an electrical current and (ii) new electrical currents induced by a spin current in systems possessing specific interfaces between two different materials. Some of these spin (electrical) currents can be reversed near the interface by reversing the applied electrical (spin) current. Similarities and differences between these novel spintronic effects and the well-known spin Hall and inverse spin Hall effects are also discussed.

cond-mat.mtrl-sci↗
arXiv2013

Detection of the microwave spin pumping using the inverse spin Hall effect

We report electrical detection of the dynamical part of the spin pumping current emitted during ferromagnetic resonance (FMR) using the inverse Spin Hall Effect (ISHE). The experiment is performed on a YIG$|$Pt bilayer. The choice of YIG, a magnetic insulator, ensures that no charge current flows between the two layers and only pure spin current produced by the magnetization dynamics are transferred into the adjacent strong spin-orbit Pt layer via spin pumping. To avoid measuring the parasitic eddy currents induced at the frequency of the microwave source, a resonance at half the frequency is induced using parametric excitation in the parallel geometry. Triggering this nonlinear effect allows to directly detect on a spectrum analyzer the microwave component of the ISHE voltage. Signals as large as 30 $μ$V are measured for precession angles of a couple of degrees. This direct detection provides a novel efficient means to study magnetization dynamics on a very wide frequency range with great sensitivity.

cond-mat.mes-hall↗
arXiv2013

Conduction of spin currents through insulating oxides

Spintronics is a field of electronics based on using the electron spin instead of its charge. The recent advance in the manipulation of pure spin currents, i.e. angular momentum transfer not associated to conventional charge currents, has opened new opportunities to build spin based devices with low energy consumption. It has also allowed to integrate ferromagnetic insulators in spintronic devices, either as spin sources or spin conductors using their magnetic excitations to propagate a spin signal. Antiferromagnetic insulators belong to another class of materials that can also sustain magnetic excitations, even with a higher group velocity. Hence, they have potential as angular momentum conductors, possibly making faster spin devices. At the opposite end, angular momentum insulators are also required in spintronic circuits. The present letter underlines some essential features relevant for spin current conduction, based on measurements of angular momentum transmission in antiferromagnetic NiO and in the non-magnetic light element insulator SiO$_2$.

cond-mat.mtrl-sci↗
arXiv2013

Comparative Measurements of Inverse Spin Hall and Magnetoresistance in YIG|Pt and YIG|Ta

We report on a comparative study of spin Hall related effects and magnetoresistance in YIG|Pt and YIG|Ta bilayers. These combined measurements allow to estimate the characteristic transport parameters of both Pt and Ta layers juxtaposed to YIG: the spin mixing conductance $G_{\uparrow \downarrow}$ at the YIG$|$normal metal interface, the spin Hall angle $Θ_{SH}$, and the spin diffusion length $λ_{sd}$ in the normal metal. The inverse spin Hall voltages generated in Pt and Ta by the pure spin current pumped from YIG excited at resonance confirm the opposite signs of spin Hall angles in these two materials. Moreover, from the dependence of the inverse spin Hall voltage on the Ta thickness, we extract the spin diffusion length in Ta, found to be $λ_{sd}^\text{Ta}=1.8\pm0.7$ nm. Both the YIG|Pt and YIG|Ta systems display a similar variation of resistance upon magnetic field orientation, which can be explained in the recently developed framework of spin Hall magnetoresistance.

cond-mat.mes-hall↗
arXiv2008

Giant orbital moments are responsible for the anisotropic magnetoresistance of atomic contacts

We study here, both experimentally and theoretically, the anisotropy of magnetoresistance in atomic contacts. Our measurements on iron break junctions reveal an abrupt and hysteretic switch between two conductance levels when a large applied field is continuously rotated. We show that this behaviour stems from the coexistence of two metastable electronic states which result from the anisotropy of electronic interactions responsible for the enhancement of orbital magnetization. In both states giant orbital moments appear on the low coordinated central atom in a realistic contact geometry. However they differ by their orientation, parallel or perpendicular, with respect to the axis of the contact. Our explanation is totally at variance with the usual model based on the band structure of a monatomic linear chain, which we argue cannot be applied to 3d ferromagnetic metals.

cond-mat.other↗